1. Technical Field
The present invention pertains to the field of wireless communication. More particularly, the present invention pertains to wireless retransmission of data in response to an error in transmission.
2. Discussion of Related Art
For the evolution of 3G systems such as WCDMA, other access techniques such as OFDM have been considered besides WCDMA, especially for the downlink. Also, for future systems such as 3.9G and 4G, OFDM is a strong candidate for the access technique. OFDM uses subcarriers at different frequencies separated from adjacent frequencies so that each subcarrier is (at least approximately) orthogonal the other subcarriers. The information to be communicated, sometimes as packets (as opposed being communicated via a circuit switched network), is simultaneously conveyed by typically several of the subcarriers. In case of downlink, where the signal being transmitted includes packets for more than one user, the packet information for a first of the users is simultaneously conveyed by a first set (typically neighboring) of the subcarriers, and the packet information for a second of the users is simultaneously conveyed by a second set (typically neighboring) of the subcarriers, and so on. This approach relies on separation of the users in the frequency domain. Another way of separating the users could be in the time domain, such that different users have access to the frequency bands at different times.
In typical OFDM-based systems, the subcarriers are sometimes grouped into scheduling units, known as resource pools in 3GPP. The advantage of this approach—grouping subcarriers—is in connection with signalling complexity. A user needs to be told which resource pools to receive, and so having a large number of resource pools or scheduling units increases signalling complexity. Due to multipath propagation, the received signal power in each frequency band of an OFDM signal (i.e. the power for each subcarrier) often varies as a function of time (often as a result of the user moving during a communication session). Due to such power variations the corresponding channel quality of each subcarrier also varies. To combat the effects of multipath propagation, such a system will typically use forward error correction techniques. One of the most often-used techniques for forward error correction is turbo coding.
To be able to increase the spectral efficiency of such a system, the scheduling algorithms—used for scheduling when packets will be communicated—will typically use an aggressive approach where the packet error probability is relatively high (5-30%), and will rely on hybrid automatic repeat request (H-ARQ) to recover erroneously received packets.
One problem in using turbo coding is that for optimum decoding performance, all bits should be received with the same error probability. Having a high degree of variation in the error probabilities among the received bits causes packet error performance to degrade significantly. To randomize the occurrence of errors among the received bits, an interleaver is typically used (so that sequences of bits having higher error probability is largely avoided). If H-ARQ is used, the bits for retransmission will in some instances (depending on the retransmission strategy) be placed in the same positions in the bit stream (and thus the same position after the interleaving). This, in conjunction with scheduling based on segmentation into frequency sub-bands (bands of sub-carriers), can cause retransmitted data to have the same errors as in the original transmission.
What is needed is some way to avoid retransmitting data using OFDM so as to be likely to have the same errors as originally.